For signal beam-forming in radar and wireless communication systems, a multi-channel phase coherent signal generator is needed to generate the multi-channel signals with controlled phase difference and a multi-channel phase coherent signal analyzer is needed to capture the multi-channel signals with controlled phase difference.
Current approaches usually adopt the following methods: 1) Multiple RF phase shift components; 2) Use DDS (Direct Digital Synthesizer) to generate multiple carriers with different phases; and 3) Baseband digital phase shifting.
The disadvantage of the multiple RF phase shift approach is that the phase shifter typically has a limited frequency range. It cannot cover a broad frequency range which is required for 5G applications and the phase control resolution is typically limited.
The advantage using a DDS is that it can support very fine phase adjustment. However, the disadvantage is that it is very challenging to support high carrier frequency by DDS directly, and DDS phase shifting is typically applied to a continuous waveform (CW) signal not a broadband modulated signal. The DDS signal typically suffers from a lot interference as well. For the baseband digital phase shifting approach, it can generate very fine phase control, support broadband modulated signal and also cover a very wide frequency span depending on the RF up conversion and down conversion capability. However, it requires a whole chain including RF down conversion, digitization, phase adjustment, digital to analog conversion and up conversion etc. to achieve the phase control. It is thus very expensive especially when multi-channel broadband signal generation/analysis capabilities with fine phase control are required for a very wide frequency span.
The use of multiple channel phase coherent instruments and baseband adjustment to generate multi-channel broadband signals and/or capture multi-channel broadband signals with fine phase control is a variety of the third approach and thus share the same advantages and disadvantages.
For a radar test and/or for a 5G massive multiple-in-multiple-out (MIMO) test, for example, a cost effective multi-channel, broad bandwidth signal generation approach which can cover a very wide frequency span is needed.